Portable electronic devices such as broadcast radio receivers and radiotelephones are continually driven to smaller sizes while offering better performance and more varied features. One engineering challenge in continual size reduction has been improved antenna designs to ensure a reliable and efficient communications interface between the portable device and other entities with which it communicates. Antennas serve to transition an energy signal between circuitry and a wireless channel. The effectiveness of energy transfer between an antenna and its adjacent circuitry is dependent on the terminal impedance of the antenna and that of the adjacent circuit over the desired frequency range.
The directive properties of some antennas, exhibited as a non-uniform radiation pattern, give rise to the concept of antenna gain. Gain is the measure of radiation intensity in a given direction as compared to radiation intensity if the radiation pattern were uniform in all directions. Gain is principally dependent upon the size of an antenna, generally expressed in wavelengths. The gain is measured either on a linear scale or logarithmically in decibels. Larger antennas generally exhibit higher gain. When one or more dimensions of an antenna is significantly larger than a wavelength, its radiation pattern defines a lobe structure, generally with one or more maximums flanked by sidelobes. Maximizing the lobe structure for the intended reception has led to many different configurations for antennas.
A loop antenna has one or more closed conductive pathways, and sometimes a fixed or variable capacitor coupled to the terminals or in series along the conductor of the loop to tune the antenna to resonance. Loop antennas are desirable for their broadband capabilities, a regime in which portable devices are entering using a standard entitled digital video broadcast for handheld devices (DVB-H). Loop antennas are directional, exhibiting radiation patterns with a very large main lobe and a high gain in the direction of that main lobe. By their nature, portable electronic devices are suited to non-directional antennas, since a tower with which the portable device communicates may or may not be aligned with the main lobe of a directional antenna at any given time. Efficiency in a loop antenna is proportional to the area of the loop. In the prior art, a loop antenna no larger than the size of a handheld device results in a very inefficient antenna, which needs additional lossy matching circuits and increases necessary signal processing in the portable device and adds cost while simultaneously reducing battery life.
What is needed in the art is a broadband antenna that is sufficiently small and lightweight for use with a portable device, and that exhibits good reception properties over a broadband frequency range with acceptable efficiency.